The present invention relates to a clutch changeover circuit for a non-stage transmission, and more particularly to a clutch changeover circuit suitable for use in a multi-stage type stationary hydraulic mechanical non-stage transmission.
As a multi-stage type stationary hydraulic mechanical non-stage transmission, the one having the arrangement shown in FIG. 1 is generally known. In this transmission, an input shaft 1 and an output shaft 2 are connected to each other via the following: a planetary gear P1 for accelerating the mechanical input of the second speed, a synthetic planetary gear P2 for a mechanical drive system and a hydraulic drive system for the second and fourth speeds, and a synthetic planetary gear P3 for a mechanical drive system and a hydraulic drive system for the first, second, third, and fourth speeds. A variable-discharge-capacity pump 3 driven via gears 5, 6, 7 as well as a volume-fixing motor 4 driven by this pump 3 are connected to the input shaft 1. The output of the pump 3 is transmitted to a sun gear 52 of the synthetic planetary gear P2 via gears 8, 9, 10 and to a sun gear 53 of the synthetic planetary gear P3 via gears 8, 9, 11, 12. These gears are provided with clutches for changing over the speed stage. Specifically, a clutch C1 for the first speed is provided for connecting or disconnecting the rotation of a ring gear R3 of the synthetic planetary gear P3 on the output shaft 2 side, and a clutch C2 for the second speed is provided for connecting or disconnecting the rotation of a ring gear R1 of the planetary gear P1 on the input shaft 1 side. Furthermore, a clutch C3 for the third speed for connecting or disconnecting the transmitted rotation from the input shaft 1 is provided on a shaft member 13 which connects together the ring gears of the synthetic planetary gear P2 and the synthetic planetary gear P3, while a clutch C4 for the fourth speed for connecting or disconnecting the transmitted rotation from the input shaft 1 is provided on an arm 14 which connects together the planetary gears of the planetary gear P1 and the synthetic planetary gear P2.
In such a transmission, since a speed change is effected at a point where there is no difference in the relative speed with respect to the following-stage clutch, it is possible to change the speed-change stage in which there is no shock which is caused by the changeover of the clutch.
In addition, the speed change is always effected in the order of the respective adjacent speed stages and is not effected by jumping a midway speed stage.
A hydraulic circuit such as the one shown in FIG. 2 (Japanese Utility Model Application No. 151193/1984) is used to effect a changeover among the clutches C1, C2, C3, and C4 with respect to the above-described transmission. This circuit is so arranged that oil circuits for the respective clutches C1, C2, C3, and C4 are provided to a hydraulic pump 101 in parallel, and pressure regulating valves 111, 121, 131, 141, reducing valves 112, 122, 132, 142, and clutch changeover valves 113, 123, 133, 143 are disposed in these oil passages in the order mentioned from the upstream side, the clutch changeover valves being respectively opened and closed by solenoid valves 114, 124, 134, 144. Hydraulic pressure from the hydraulic pump 101 acts on each of the solenoid valves via a reducing valve 102 serving as a pilot valve, the clutch changeover valve is actuated upon activation of the solenoid valve, and a corresponding clutch is adapted to operate. Hydraulic oil from the hydraulic pump 101 is returned to the tank 100 via a pressure regulating valve 103.
In accordance with such a hydraulic circuit, since the pressure regulating valves 111, 121, 131, 141 are provided in the oil circuits leading to the respective clutches C1, C2, C3, C4, it is possible to prevent the pressure of the presently engaged clutch from falling, until a following-stage clutch is engaged during a speed change. Hence, it is possible to obtain an advantage in that the clutch engaging period can be minimized with respect to the flow rate of the hydraulic pump. Accordingly, when an attempt is made to engage the clutch C2 from the state in which the clutch C1 is engaged, if the oil passage to the clutch C2 is opened, the pressure regulating valve 103 placed in a supply oil passage 200 from the hydraulic pump 101 is closed due to a drop in the circuit pressure. However, since the pressure regulating valve 121 is actuated, the pressure within the supply oil passage 200 is maintained above a set pressure of the regulating valve 121, so that the pressure of the clutch C1 is maintained at a set pressure of the regulating valve 121.
However, with the conventional clutch changeover circuit, although the pressure within the supply oil passage 200 is maintained above a set pressure of the regulating valve concerned, there has been a drawback which stems from the fact that the override characteristics of the regulating valves are good. For instance, when an attempt is made to effect a changeover operation between, for instance, the clutches C1 and C2, it is assumed that, as shown in FIG. 3, the pressure gradients of the regulating valves 111, 121, and the regulating valve 103 disposed in the supply oil passage 200 are small with respect to a flow rate Q of the pump, and that the set pressures P111, P121 of the clutch-side regulating valves 11, 121 fluctuate vertically, as illustrated in the drawing, due to production errors or the like (P111&gt;P121). If the clutch C2 is operated in such a state, even if oil flows from the hydraulic pump 101 at a full flow rate Q.sub.A, the supply pressure rises only up to the full bore hydraulic pressure P.sub.B of the regulating valve 121, so that the pressure does not reach the set pressure of the C1 clutch-side regulating valve 111. In such a case, the hydraulic oil does not flow to the clutch C1, and since leakage usually occurs downstream of the regulating valve, the pressure fails to rise, and the pressure of this clutch hence declines. Consequently, there has been a problem in that this can result in the sliding of the clutch, ultimately resulting in the possible damage to the clutch. For this reason, it has conventionally been necessary to positively use regulating valves having poor override characteristics, as shown in FIG. 4. In this case, since the pressure gradients with respect to the flow-rate Q of the pump are large, the characteristic line of the full bore hydraulic pressure P.sub.B on the side of the clutch to which a changeover is to be effected intersects the characteristic line of the set pressure P111 of the regulating valve on the side of the already operating clutch (P.sub.B &gt;P111) As a result, the line pressure acts on the already operating clutch, so that no damage to the clutch results even if the oil may leak.
Thus, it has hitherto been impossible to use regulating valves having good override characteristics, and there has been the drawback that regulating valves having poor override characteristics have to be used positively, which has been a large disadvantage. In addition, in correspondence with the poor override characteristics, it has been necessary to set the pressure of the regulating valve 103 provided in the supply oil passage 200 at a high level, which disadvantageously necessitates the capacity of the hydraulic pump 101 to be increased.
Furthermore, with the conventional clutch changeover circuit, it has been necessary to provide pressure regulating valves separately in oil passages leading to the clutches. This has brought about the problem of the circuit configuration becoming complicated and the valve mechanisms incorporated in this circuit becoming large in size.